1. Field of the Invention
This invention relates generally to telecommunications, and more particularly, to wireless communications.
2. Description of the Related Art
A pilot symbol based channel estimation technique may be used to obtain good and robust channel estimates. Channel estimation is useful in a packet-switched wireless communication system to determine a channel state for a packet. To perform channel estimation and synchronization, pilot symbols are usually embedded in a data stream. These pilot symbols are utilized to obtain an optimal channel estimation performance. The pilot symbols known to a receiver are multiplexed with data symbols for channel estimation.
For channel estimation, given a number of pilot symbols in a packet-based wireless communication system, training may be performed in a packet in the form of pilot symbols, allowing a rapid and accurate estimation of the channel. As an example of such pilot symbols, consider the Institute of Electrical and Electronics Engineers (IEEE) 802.11a standard, where a long pilot symbol is provided as part of a packet preamble for both frequency offset estimation and initial channel estimation.
However, channel estimation is a challenge for providing reliable wireless transmissions. With wideband code division multiple access (WCDMA) wireless communication systems, channel estimation is accomplished by using pilot symbols that are periodically inserted in a data stream of each user. Specifically, channel estimation with pilot symbols requires despreading the pilot symbols for each user and averaging over the pilot symbols within a time slot. For example, the pilot symbols are clustered in the case of the Global System for Mobile Communications (GSM) systems and are spread evenly in the data stream if a pilot symbol assisted modulation is used.
Many mobile communication systems, such as a Universal Mobile Telecommunication System (UMTS), make use of pre-defined pilot symbol sequences to estimate the channel coefficients of a wireless propagation channel in a receiver. The estimation of the channel coefficients is useful to compensate the impact of the wireless propagation channel on a transmitted signal and to allow for the coherent detection of the received symbols. Due to the pilot symbol based channel estimation, the channel estimate may be correlated with the received pilot symbol. To measure the quality of the reception, the receiver may evaluate the number of erroneously detected pilot symbols. In case the channel estimate is correlated to the pilot symbol under consideration, the error rate of the received pilot symbols is artificially reduced. Because of this bias being a difference between the detected pilot symbols over all the possible received pilot symbols and the pre-defined pilot symbols, the pilot symbol error rate may not be a good measure for a reception quality indicating a quality or fidelity of a received signal.
Various methods for estimating a channel based on received pilot symbols are known in the literature. However, the majority of channel estimation methods involve a certain correlation between the received pilot symbols and the corresponding channel estimate. Noise and/or interference may lead to errors in the detection of the pilot symbols, i.e. the receiver detects a pilot symbol, which is different from the pre-defined pilot symbol. The pilot symbol error rate increases if the ratio between signal power and noise plus interference power decreases. Hence, the detection of the pilot symbol error rate may be considered a quality indicator of the reception.
In case the correlation between the channel estimates and the received pilot symbol is not negligible, the observed pilot symbol error rate is biased. As already stated above, most of the conventional channel estimation methods have the aforementioned correlation property. In this case, the noise and interference component in the received pilot symbol impacts the channel estimate in such a way that the probability of detecting the supposedly correct pilot symbol is artificially increased. In turn, the detected pilot symbol error rate is biased towards a lower error rate than the actual pilot symbol error if the channel estimation is not biased. FIG. 2 shows a conventional transmission model 200 without bias removal. The transmission model 200 may be operating on a flat fading channel with a single receive antenna. A pilot symbol error rate bias in the transmission model 200 is not limited to this model, but it also applies to various other kinds of receiver structures. The input to a pilot symbol detector 205 is biased towards the pilot symbol p(k). This bias lowers the perceived pilot symbol error rate, which limits the usability of this measurement as a quality indicator for the reception.
The present invention is directed to overcoming, or at least reducing, the effects of, one or more of the problems set forth above.